1. Field of the Invention
This invention relates to DC-to-DC converters and more specifically to a DC-to-DC converter having a single-fault tolerant clamp for sinking excess current generated by motor-generator loads.
2. Description of the Related Art
Electrical systems typically have a primary bus that provides a well regulated supply voltage. Often times, this voltage is generated externally and routed to the primary bus. Alternately, the primary voltage may be generated on board the particular electrical system. For example, spacecraft employ large solar arrays to generate the supply voltage.
These electrical systems typically include a variety of subsystems, i.e. loads, that operate from different supply voltages. Therefore, the primary voltage must be converted to one or more secondary supply voltages, which should exhibit minimal ripple and be insensitive to changes in load current. The mechanism for making this conversion is called a DC-to-DC converter, which receives the primary voltage and sources current to supply the load connected to a particular secondary bus while regulating the secondary bus voltage. If the bus voltage should fall too low, one or more of the loads may degrade or turn off completely. Similarly, if the bus voltage should increase too much, one or more of the loads may become damaged.
One type of DC-to-DC converter is called a switching regulator. A transistor operated as a saturated switch periodically applies the full primary voltage across an inductor for short intervals. As the inductance builds up, energy is stored in the inductor's magnetic field and then transferred to a filter cap that smooths the secondary bus voltage. An error amplifier compares the secondary bus voltage to a set point voltage and outputs a voltage control signal. The control signal is compared to a ramp voltage, which incorporates the present amount of source current, to control the pulse width that is applied to the transistor.
The pulse width determines the amount of current sourced by the switching regulator to regulate the secondary bus voltage. As a result, when the loads collectively demand additional current the bus voltage dips temporarily, which causes the control loop to drive the regulator harder to source more current and pull the bus voltage up to the set point. Similarly, when the loads collectively demand less current the bus voltage rises temporarily which reduces the control signal causing the switching regulator to source less current and maintain regulation. The well known "fly-back" topology is one type of switching regulator that is simple, low power and provides excellent primary-to-secondary bus isolation.
In any given application, a particular secondary bus may supply a number of loads such as amplifiers that exclusively consume power, i.e. sink current. A motor-generator type load such as a reaction wheel used for satellite attitude control or momentum wheels consumes power (sinks current) from the secondary bus when motor speed is increasing and generates power (sources current) when motor speed is reduced. Systems that include motor-generator type loads have the potential of imparting energy back into the DC-to-DC converter, which would cause the secondary bus voltage to increase rapidly. This may cause electrical overstress to other loads connected to the secondary bus.
To avoid this potentially catastrophic problem, known bus architectures are designed such that the maximum sourcing capacity of the motor-generator type loads is less than the fixed amount of current sunk by other loads. Thus, as the motor-generator loads source more current, the DC-to-DC converter can reduce the amount of current it sources thereby maintaining a proper current balance and regulating the secondary bus voltage. If this design criteria were violated and if the total current sourced by the motor-generator loads were to exceed the total current sunk by the other loads then, since the most known DC-to-DC converters can do is to stop sourcing current, the excess current would be driven back into the output of the DC-to-DC converter causing the secondary bus voltage to increase.